[whatwg] Add 'type' attribute to <mark>

[Pentasis]

>
> The spec already describes how to do footnotes:
>
>   http://www.whatwg.org/specs/web-apps/current-work/#footnotes
>

Yes, but this is a theoretical explanation that does not provide a 
consistent, practical solution.

>
>> As the <mark> element has different usages defined on it already why not
>> include a "type" attribute (or similar) that defines what it is used
>> for. One of these types would then be "footnote", others could be
>> (relating to what is already in the spec) "term", "highlight" etc. (I am
>> sure others would be much better at thinking up names than I am).
>
> That's what the "class" attribute is for.
>

Like I said before, I think class (and id) should be reserved for styling 
and not be content/semantic related at all. It is strange how we all 
advocate separation of the two but allow these attributes to encompass 
descriptive values for both.

Bert